The invention relates to a piezoelectric element which becomes deformed upon receipt of a supplied drive signal, to a piezoelectric actuator and a liquid ejecting head incorporating the piezoelectric element as a drive source.
A piezoelectric element is formed from piezoelectric ceramics or a piezoelectric macromolecular film utilizing a high molecular compound and becomes deformed upon receipt of supplied electric energy. Here, the piezoelectric ceramics is formed by compacting and sintering metal oxide powder, such as BaTiO3, PbZrO3, PbTiO3, which are piezoelectric materials and exhibit a piezoelectric effect. The piezoelectric element has been in widespread use as a drive element for, e.g., a liquid ejecting head, a micropump, and a sounding body (a speaker or the like). Here, the liquid ejecting head ejects a droplet from a nozzle orifice by inducing pressure fluctuations in a liquid stored in a pressure chamber. The liquid ejecting head is embodied as, e.g., a recording head to be used in an image recording apparatus such as a printer, a liquid-crystal ejecting head for use in manufacturing a liquid-crystal display, and a coloring material ejecting head to be used for manufacturing a color filter. Here, the micropump is an ultrasmall pump capable of ejecting a very small volume of liquid and used at the time of, e.g., delivery of a trace amount of chemical.
One of the important components used in such a liquid ejecting head and a micropump is a piezoelectric actuator formed by placing a piezoelectric element on the surface of a vibration plate. The piezoelectric actuator is mounted on a pressure chamber formation substrate having a void which is to serve as a pressure chamber, and a portion of the pressure chamber is partitioned by the vibration plate. When ejection of a droplet or delivery of liquid is to be performed, a drive pulse is supplied to the piezoelectric element, to thereby deform the piezoelectric element and the vibration plate (i.e., the deformed portion of the pressure chamber) and change the volume of the pressure chamber.
In the field of the liquid ejecting head and that of the micropump, strong demand exists for high-frequency driving of the piezoelectric element. This demand is intended for implementing high-frequency ejection of a droplet and enhancing liquid delivery capability. In order to implement high-frequency driving of the piezoelectric element, the compliance of the deformed portion must be made smaller than that of a related-art piezoelectric element and the extent to which the piezoelectric element is deformed must be made greater than that to which the related-art piezoelectric element is deformed. The reason for this is that a reduction in the compliance of the deformed portion results in enhancement of responsiveness, and hence driving of the piezoelectric element at a frequency higher than that required conventionally becomes possible. Another reason is that an increase in the extent to which the piezoelectric element is deformed results in an increase in volumetric change in the pressure chamber, and hence the volume of droplet to be ejected or the volume of droplet to be delivered can be increased.
A piezoelectric element of multilayer structure has been proposed for sufficing for a characteristic pertaining to the compliance of the deformed portion and a characteristic pertaining to the extent to which the piezoelectric element becomes deformed, the characteristics being mutually contradictory. For example, piezoelectric elements disclosed in Japanese Patent Publications Nos. 2-289352A (page. 6; FIG. 5) and 10-34924A (page.5; FIG. 9) are formed from a piezoelectric layer having a two-layer structure: that is, an upper layer piezoelectric substance and a lower layer piezoelectric substance. Drive electrodes (individual electrodes) are formed at a boundary between the upper layer piezoelectric substance and the lower layer piezoelectric substance. A common electrode is formed on an outer surface of the upper layer piezoelectric substance, and another common electrode is formed on an outer surface of the lower layer piezoelectric substance.
In the case of the piezoelectric element of multilayer structure, the drive electrodes are provided at the boundary between the upper layer piezoelectric substance and the lower layer piezoelectric substance. Hence, an electric field, whose intensity is determined by an interval between the drive electrodes and the common electrodes (i.e., the thickness of each piezoelectric substance) and by a potential difference between the drive electrodes and the common electrodes, is imparted to the piezoelectric substances of respective layers. Therefore, in contrast with a piezoelectric element of monolayer structure formed by interposing a single layer piezoelectric substance between the common electrode and the drive electrodes, the piezoelectric element can be deformed at the same drive voltage as that conventionally required, even when the compliance of the deformed portion is reduced by increasing the entire thickness of the piezoelectric element to some extent.
However, characteristics capable of responding to recently-growing demand cannot be achieved by mere use of the piezoelectric element of multilayer structure. Therefore, users are forced to use, as an actual product, a piezoelectric element of monolayer structure formed by interposing a single layer piezoelectric substance between a common electrode and drive electrodes. Various factors are conceivable as being responsible for this, and insufficient efficiency of deformation of the piezoelectric element can be conceived as one such factor.